Changes in benthic microfossil assemblages before, during and after the early Toarcian biotic crisis in the Portland-Wight Basin (Kerr McGee 97/12-1 well, offshore southern England)

Abstract

The uppermost Pliensbachian (Margaritatus Zone) - upper Toarcian (Pseudoradiosa Zone) sediments (silty and variably calcareous claystones, marlstones, silty sandstones, and sandy limestones) recovered from the Kerr McGee 97/12–1 well were deposited in the Portland-Wight Basin. The foraminiferal assemblages of these sedimentary succession are dominated by calcareous forms of the suborder Lagenina and locally Robertinina. Three main ecostratigraphic intervals have been identified, based on changes in the foraminiferal and ostracod assemblages that occurred before, during and after the sedimentation of the earliest Toarcian black, organic-rich, non-calcareous claystones. The ecostratigraphic interval I (upper Pliensbachian) is characterized by a diverse assemblage with specialist, opportunist and intermediate forms, but dominated by Lenticulina muensteri. The ostracods in this interval are dominated by Metacopina. This assemblage is interpreted to represent a phase that pre-dates the biotic crisis. The ecostratigraphic interval II, related to the deposition of the lower Toarcian black claystones, shows an abrupt increase in the opportunist form Reinholdella (91%) with very low diversity and a high abundance of specimens, mainly Reinholdella macfadyeni and R. dreheri. In this interval the ostracods Metacopina dissapear. The top of the black claystones is almost barren of foraminifera and ostracods (only two specimens recorded of Trochammina eoparva). This second interval represents the biotic crisis phase related to the Jenkyns Event. The ecostratigraphic interval III (upper part of the lower Toarcian through to the upper Toarcian) is characterized by an increase in the diversity and abundance of foraminifera, as well as the dominance of Lenticulina. The ostracod assemblage shows a turnover with respect to the upper Pliensbachian assemblage with dominance of Cytherocopina, Platycopina and Bairdiocopina. This assemblage is interpreted to reflect the increased availability of oxygen from deep-infaunal to epifaunal microhabitats and the return to normal conditions at the sea floor. The incidence of the global warming related to the Jenkyns Event in the biotic crisis is also considered. Ecostratigraphic fluctuations in microfossil assemblages from the Kerr McGee 97/12–1 well across the T-OAE event are comparable with those described from the Mochras Borehole (west Wales), the Lusitanian Basin (Portugal), and the Atlassic Basin (North Africa).